1. Field of the Invention
The present invention relates to achieving a higher S/N for readout signals of an optical disk apparatus.
2. Background Art
With the commercialization of Blu-ray Disc, which uses a blue laser diode and a high-NA objective lens, optical disks have more or less reached a limit in terms of the resolution of an optical system. The multi-layering of recording layers is viewed as holding promise in further increasing capacity in the future. With respect to such multi-layered optical disks, it is necessary that the detected quantity of light from each recording layer be substantially equal. Thus, the reflectivity from certain recording layers must be made smaller. However, along with the increase in the capacity of optical disks, data transfer speed also continues to increase due to the need for higher dubbing speed for video and the like. Thus, it is becoming difficult to secure an adequate S/N ratio for readout signals as is. Therefore, in order to simultaneously advance the multi-layering of recording layers and the increase in speed in the future, achieving higher S/N for detected signals becomes a requirement.
Techniques relating to achieving higher S/N for readout signals of optical disks are described, for example, in Patent Document 1, Patent Document 2, Patent Document 3, and the like. Patent Document 1 and Patent Document 2 relate to achieving higher S/N for readout signals of magneto-optical disks. They divide light from a semiconductor laser before it is irradiated on an optical disk. The light that is not irradiated on the optical disk is combined and made to interfere with light reflected from the optical disk. They thus aim to amplify the amplitude of weak signals by increasing the quantity of light of the light that is not irradiated on the optical disk. In differential detection of light transmitted and reflected by a polarization beam splitter that is conventionally used in signal detection for magneto-optical disks, detection is performed essentially by causing interference between an original incident polarization component and a polarization component, which occurs due to polarization rotation caused by a magneto-optical disk and which is orthogonal to the incident polarization direction, and by amplifying the orthogonal polarization component with the incident polarization. Therefore, signals can be amplified if the original incident polarization component is amplified. However, the light intensity that is incident on an optical disk must be kept at or below a certain level so that data is not erased or overwritten. In contrast, in the related art described above, a signal light and interference light are separated in advance. The latter is made to interfere with the signal light without being focused on the disk so that the intensity of the light that is made to interfere for signal amplification can be increased irrespective of the light intensity on the surface of the disk. Thus, in principle, the more the intensity is increased within a range that the light intensity would permit, the more the S/N ratio can be increased compared to the noise of an amplifier that carries out voltage conversion of a photoelectric current from a photodetector. Patent Document 3 relates to achieving higher S/N for readout signals of optical disks employing a photochromic medium. As with Reference Document 1 and Reference Document 2, it aims to amplify signals by having light that is not irradiated on an optical disk interfere with reflected light from the optical disk. With respect to optical disks employing a photochromic medium, too, degradation of the medium is faster the higher the intensity of the incident light for signal readout is. Therefore, there is a limit to the intensity of the light irradiated on the recording medium as with the magneto-optical disks mentioned above.
In Patent Document 1, two lights are made to interfere, and the interference light intensity is detected. In so doing, the optical path length of the disk reflected light that is made to interfere is made variable so as to secure interference signal amplitude. In Patent Document 2 and Patent Document 3, differential detection is also performed in addition to interference light intensity detection. Thus, the intensity component of any light that does not contribute to the signal is cancelled out, and signal amplitude is doubled, thereby achieving higher S/N. In the differential detection of these cases, a non-polarization beam splitter is used.    [Patent Document 1] JP Patent Publication (Kokai) No. 5-342678 A (1993)    [Patent Document 2] JP Patent Publication (Kokai) No. 6-223433 A (1994)    [Patent Document 3] JP Patent Publication (Kokai) No. 6-068470 A (1994)